I have an invite I could share if someone needs one -- PM me.
Atom's better than I'd expected from the earlier 7 pages of replies in this topic. That's to be expected -- they're iterating every few days, working on responsiveness, they say. Open speed isn't in ST3's class, but on this five year old MBP 17" with 8GB, performance is fine editing the files I edit. Atom's community seems vibrant right now, with an early focus on dragging visual experience from TextMate and Sublime via theme and syntax development. It seems the Github folk built tools to make that efficient. I'm sure it'll take longer for the heavier functions to arrive. For example, right now they have nothing like my beloved SFTP, SubblimeGit, and Reg_Replace packages. That'll happen in time.
Atom has some velocity right now -- and is a very viable tool. ST3 is a great editor as it stands right now, and will stay that way as long as the environments that support it don't change out from under it. It also has great plugin authors.
But I also think Sublime HQ would do well to reignite a sense of velocity behind its creation in light of both user preference and dynamic environment.